Corrosion protected, multi-layer fuel cell interface

ABSTRACT

An improved interface configuration for use between adjacent elements of a fuel cell stack. The interface is impervious to gas and liquid and provides resistance to corrosion by the electrolyte of the fuel cell. The multi-layer configuration for the interface comprises a non-cupreous metal-coated metallic element to which is film-bonded a conductive layer by hot pressing a resin therebetween. The multi-layer arrangement provides bridging electrical contact.

BACKGROUND OF THE INVENTION

The present invention relates to improved elements for use in fuel cellstacks, and more particularly, to a stack having a corrosion resistant,electrically conductive, fluid impervious interface member therein.

It has been known for some time that fuel cells and stacks of such cellscan be extremely advantageous as power supplies, particularly forcertain applications such as a primary source of power in remote areas.It is highly desirable that any such fuel cell assembly be extremelyreliable. Various fuel cell systems have been devised in the past toaccomplish these purposes. Illustrative of such prior art fuel cells arethose shown and described in U.S. Pat. Nos. 3,709,736, 3,453,149 and4,175,165. A detailed analysis of fuel cell technology comparing anumber of different types of fuel cells appears in the "EnergyTechnology Handbook" by Douglas M. Consadine, published in 1977 byMcGraw Hill Book Company at pages 4-59 to 4-73.

U.S. Pat. No. 3,709,736, assigned to the assignee of the presentinvention, describes a fuel cell system which includes a stackedconfiguration comprising alternating fuel cell laminates andelectrically and thermally conductive impervious cell plates. Thelaminates include fuel and oxygen electrodes on either side of anelectrolyte comprising an immobilized acid electrolyte. U.S. Pat. No.3,453,149, assigned to the assignee of this invention, is illustrativeof such an immobilized acid electrolyte.

In U.S. Pat. No. 4,175,165, assigned to the assignee of the presentinvention, a stacked array of fuel cells is described wherein gasdistribution plates include a plurality of gas flow channels or grooves,with the grooves for the hydrogen gas distribution being arrangedorthogonally relative to the grooves for the oxygen distribution. Thegas distribution plates themselves, whether they are individualtermination plates for one or the other of the gases, or bi-polar platefor distributing both gases in accordance with this disclosure, areformed of an electrically conductive impervious material.

In larger stacks of fuel cells, heat dissipation from the cell'soperation becomes a consideration. To solve this problem, cooling cellshave been employed in the stack to maintain the thermal balance of thestack. These cooling cells have frequently been made of a metal such asaluminum. Metal plates have also been utilized for the currentcollection element in fuel cell stacks.

One problem which arises with respect to both the cooling plates and thecurrent collecting plates in a fuel cell stack is that they are subjectto corrosion attack by the acid electrolyte. In order to preventcorrosion, an interface layer has been utilized comprising a conductivecarbon layer, such as Grafoil (manufactured by Union CarbideCorporation), and a copper screen arranged between the cooling orcurrent collecting plate and the next termination plate. The interfacelayer can be a highly rolled, densely-packed, carbon, fibrous materialwhich is at least partially resistant to acid attack.

More recently, in patent application Ser. No. 06/597,559, filed on Apr.6, 1984 by Kaufman et al, now U.S. Pat. No. 4,526,843 assigned to theassignee of the present invention, the interface layer comprises twoconducting layers, one of which is perforated, bonded together and tothe metal element by resin hot-pressed between the two conductinglayers. While the interface configuration provided is an advance overthe prior art in improving corrosion resistance, there is a continuingneed to further improve corrosion resistance while keeping themanufacturing and maintenance costs of the stacks as low as possible,given the foregoing considerations in fuel cell stack designs.

Accordingly, this invention provides an improved interface configurationbetween elements of a fuel cell stack. The interface configuration isimpervious to gas or liquids so as to impart resistance to corrosion bythe electrolyte and to provide good electrical and thermal conductivity.A fuel cell stack is also provided which includes at least one of saidimproved interface configurations.

A process for making the improved interface configuration and fuel cellstack as above is also provided.

SUMMARY OF THE INVENTION

In accordance with this invention, an improved interface configurationis provided for use between elements of a fuel cell stack. The interfaceconfiguration is gas and liquid impervious to resist migration of theacid electrolyte used in the cell which could cause corrosion. Corrosionproducts can communicate back to and impair the operation of or poisonthe catalyst at the fuel cell electrodes. It is also essential that thevarious elements in the fuel cell stack be electrically interconnectedto provide bridging electrical contact between adjacent elements. Theinterface configuration, in accordance with this invention, is usefulbetween a gas distribution plate and an adjacent current collectingplate or between a gas distribution plate and an adjacent cooling plate.

A fuel cell stack is provided having a plurality of stacked elementswherein at least one corrosion resistant electrically conductiveinterface is arranged between two of said elements which are adjacent toone another. The interface comprises a non-cupreous metal coating formedon at least one of the two elements and a conductive layer bondedthereto by a hot-pressed resin. The conductive layer and the resin arearranged to provide bridging electrical contact between a gasdistribution plate and a current collecting plate or cooling plate, theresin filling substantially any pores in the conductive layer.

In accordance with the invention, the process for forming the interfaceconfiguration between adjacent elements of the fuel cell stack comprisesforming a metal coating on a current collecting plate or a coolingplate, and arranging between the current collecting plate or coolingplate and a gas distribution plate a conductive layer and a resin layerbetween the conductive layer and the metal-coated plate. This compositeis hot-pressed together thereby causing the resin to flow into the poresin the conductive layer without adversely affecting electricalconductivity of the layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by reference to the followingdrawings and description in which like elements have been given commonreference numbers:

FIG. 1 is a schematic representation of a fuel cell assembly comprisinga plurality of stacked fuel cells with intermediate cooling plates andterminal current collecting plates.

FIG. 2 is perspective view of a portion of the fuel cell assembly ofFIG. 1 illustrating an individual fuel cell in greater detail.

FIG. 3 is a perspective view in partial crosssection showing a corrosionresistant interface arrangement between a gas distribution plate and acooling plate.

FIG. 4 is a schematic exploded view of the arrangement forming theinterface of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An exemplary fuel cell stack assembly 10 employing a plurality of fuelcells 11 in accordance with this invention is now described withreference to FIGS. 1 and 2. Hydrogen gas input manifolds 12 are arrangedalong one side of the stack assembly 10. While a plurality of manifolds12 are shown for each group of fuel cells 11, if desired, a singlemanifold arrangement could be used. The manifolds 12 are connected to asource of hydrogen gas 14. Hydrogen gas collecting manifolds 15 arearranged along the opposing stack side in correspondence with the gasinput manifolds 12. Here again, while a plurality of manifolds 15 areshown, a single manifold could be used if desired. The collectingmanifolds 15, are, in turn, connected to a hydrogen gas dischargingsystem 17. The hydrogen gas from the input manifolds 12 flows throughgas distribution plates 18 to the collecting manifolds 15.

In a similar fashion, a plurality of oxygen or air input manifolds (notshown) are arranged along the stack side (not shown) connecting the onestack side and the opposing stack side. The oxygen manifolds areconnected to an oxygen source 19. The oxygen may be supplied in the formof air rather than pure oxygen if desired. In a similar fashion, aplurality of collecting manifolds are arranged along the stack side (notshown) opposing the stack side having the oxygen input manifolds andconnecting the respective one stack side and opposing stack side. Thesemanifolds would also be connected to an oxygen storage or recirculatingsystem (not shown). The oxygen or air from the input manifolds (notshown) flows through the oxygen gas distribution plates 20 to therespective collecting manifolds (not shown).

In this embodiment, cooling plates 21 are arranged periodically betweenadjacent fuel cells 11. Three cooling plates 21 are shown arrangedintermediate each four cell 11 array. The cooling fluid flowing throughthe cooling plates 21 is preferably a dielectric fluid, such as a hightemperature oil, such an oil being manufactured by Monsanto under thetrade name, Therminol. A pump 22 circulates the dielectric fluid viaconduit 23 and input manifold 24 into the respective cooling plates 21.The dielectric fluid then flows into collecting manifold 25 which isconnected to a heat exchanger 26 for reducing the temperature of thedielectric fluid to the desired input temperature. A conduit 27 thenconnects the heat exchanger back to the pump 22 so that the fluid can berecirculated through the respective cooling plates 21.

The fuel cells 11 and the cooling plates 21 are electrically conductiveso that when they are stacked as shown, the fuel cells 11 are beingconnected in series. In order to connect the stack assembly 10 to adesired electrical load, current collecting plates 28 are employed atthe respective ends of the stack assembly 10. Positive terminal 29 andnegative terminal 30 are connected to the current connecting plates 28as shown and may be connected to the desired electrical load by anyconventional means.

Each fuel cell 11 is made up of a plurality of elements and includes ahydrogen gas distribution plate 18 and an oxygen or air distributionplate 20. Arranged intermediate the respective gas distribution plates18 and 20 are the following elements starting from the hydrogen gasdistribution plate 18: anode 31, anode catalyst 32, electrolyte 33,cathode catalyst 34 and cathode 35. These elements 31-35 of the fuelcell 11 may be formed of any suitable material in accordance withconventional practice.

The hydrogen gas distribution plate 18 is arranged in contact with anode31. Typically, the anode comprises a carbon material having pores whichallow the hydrogen fuel gas to pass through the anode to the anodecatalyst 32. The anode 31 is preferably treated with Teflon(polytetrafluoroethylene) to prevent the electrolyte 33, which ispreferably an immobilized acid, from flooding back into the area of theanode. If flooding were allowed to occur, the electrolyte would plug upthe pores in the anode 31 and lessen the flow of hydrogen fuel throughthe cell 11. The anode catalyst 32 is preferably a platinum containingcatalyst.

The cell 11 is formed of an electrically conductive material, such as acarbon based material, except for the immobilized acid electrolyte layerwhich does not conduct electrons but does conduct hydrogen ions. Thevarious elements, 18, 31-35, and 20 are compressed together under apositive pressure. The electrolyte 33, such as phosphoric acid, isimmobilized by being dispersed in a gel or paste matrix so that the acidis not a free liquid. An exemplary electrolyte matrix could comprise amixture of phosphoric acid, silicon carbide particles and Teflonparticles.

The cathode catalyst 34 and the cathode 35 are formed of the same typesof materials as the respective anode catalyst 32 and anode 31.Therefore, the anode 31 and the cathode 35 comprise porous carbon andthe anode catalyst 32 and cathode catalyst 34 can comprise a platinumcontaining catalyst. The cathode 35 can also be treated with Teflon toprevent the electrolyte from flooding back into the porous carboncomprising the cathode.

All of the elements of the cell 11 are arranged in intimate contact asshown in FIG. 2. In order to provide an electrically interconnectedstack assembly 10, bi-polar assembly 36 is ued to connect togetheradjacent fuel cells 11. A bi-polar assembly 36 is comprised of ahydrogen gas distribution plate 18 and an oxygen or air distributionplate 20 with an impervious interface layer or plate 37 arranged betweenthem. Therefore, a bi-polar assembly 36 is comprised of the hydrogen gasdistribution plate 18 of one cell 11 and the oxygen or air gasdistribution plate 20 of the next adjacent cell 11. The interface layerof plate 37 may comprise an impervious carbon plate or any otherconventional interface as may be desired. In the bi-polar assembly 36,the respective plates 18 and 20, having the interface 37 therebetween,are securely connected together as a unit so as to have good electricalconductivity.

In order to facilitate the gas flow in the gas distribution plate 18 and20, respective channels or grooves 38 or 39 are employed. The grooves 38in the hydrogen gas distribtuion plate 18 are arranged orthogonally orperpendicularly to the grooves 39 in the oxygen or air gas distributionplate 20. This allows the grooves to be easily connected to respectiveinput and output manifolds 12 and 15, for example, on different sides ofthe cell stack assembly 10.

Although grooves within a particular plate, such as plates 18 or 19, areshown as extending in a unidirectinal manner in FIG. 2, there can becross-channels made between these grooves to aid in the distribution ofthe fluidic reactants. When such cross-channels are utilized, theprimary flow of reactants is still in the direction of the grooves 38and 39 as shown in FIG. 2; that is, in the direction that the reactantsflow between the reactant input and collecting manifolds.

The gas distribution plates 18 and 20 supply the respective hydrogen andoxygen or air gases to the surfaces of their respective anode 31 orcathode 35. In order to more evenly distribute the respective gases atthe anode 31 or cathode 35 plate surfaces, the gas distribution plates18 and 20 are preferably formed of a porous carbon material. This allowsthe respective gases to flow through the pores of the plates 18 and 20between the respective channels 38 or 39 to provide more uniform gasdistribution over the face of the respective anode 31 or cathode 35.

The current collecting plate 28 can be combined in an assembly 40 with agas distribution plate 18, as shown in FIG. 2. Since the currentcollecting plate 28 is normally formed of an impervious material, suchas aluminum, the purpose of the layer or plate 60 is to preventcorrosion of the plate 28. A cooling plate assembly, shown as 21 in FIG.1, can be made in a similar manner comprising a gas distribution plateand a cooling plate with an interface layer or plate therebetween.

Referring now to FIGS. 3 and 4, an improved interface layer 60 will bedescribed. Interface layers 60 are typically employed between gasdistribution plates 18 or 20 and cooling plates 21 or current collectingplates 28. The cooling plates and current collecting plates aregenerally formed of metal which is subject to corrosion by the acid ofelectrolyte 33.

In the prior art fuel cell stacks 10, phosphoric acid permeating from afuel cell 11 through the interface layer 60 into the areas of acopper-plated cooling plate 21 would tend to corrode it. Corrosion ofthe cooling plate 21 tends to increase the resistance of the fuel cellstack 10. In the process, corrosion products also can eventually workback to the catalyst and can poison it. The cooling plates 21 should beconductive and non-corroding; however, this is a difficult combinationto acheive. The interface layers of the prior art employed Grafoil whichcan become penetrated by the electrolyte 33 as the cell ages over thelong term. It is also an expensive material and can become more porousover time due to exposure to the electrolyte.

In accordance with the invention, a new improved interface layerconfiguration 60 for use intermediate gas distribution plates 18 or 20and cooling plates 21 or current collecting plate 28 in a fuel cellstack 10 serves to prevent corrosion by the acid of the electrolyte 33.As a result, electrical conduction through the interface 60 is preservedand poisoning of the catalyst layers 32 or 34 by corrosion products isavoided.

The improved interface layer 60 comprises a conductive layer 52preferably comprising porous carbon fiber paper, most preferablycomprising a fluoropolymer-treated carbon paper. The cooling plate 21 orcurrent collecting plate 28 is provided with a non-cupreous metalcoating (not shown), advantageously ranging in thickness from about 0.1mil to about 1.0 mil. The metal is preferably the one selected from thegroup consisting of silver, gold, ruthenium, platinum, rhodium, iridium,ruthenium-nickel or palladium-nickel or a mixture thereof. Thenon-cupreous metal coating is formed on the cooling plate 21 or currentcollecting plate 28 by an convenient technique, such as byelectroplating and the like.

Intermediate the conductive layer 52 is a layer of resin material 54.The assembly is then hot-pressed onto the metal-coated cooling plate 21or the metal-coated current collecting plate 28. The resin material 54will have filled substantially any pores in the carbon paper. Thisprocess prevents access of the electrolyte 33 or of air to theinterfacial zone. In this manner, electrical conduction is achievedwhile corrosion is inhibited or prevented.

A preferred resin material comprises polyethersulfone and the hotpressing is preferably carried out at from about 100 to about 300 psiand from about 500 degrees to about 750 degrees F. The resultantstructure is as shown in FIG. 3 wherein the carbon paper is securelybonded and electrically connected to the cooling plate 21 and compressedagainst the gas distribution plate 18.

A variety of metal coatings were evaluated for their corrosionresistance; the metals were silver, gold, ruthenium, ruthenium-nickeland palladium-nickel. All of these coatings showed better corrosionresistance as compared to copper (used in the prior art). However, inorder to ensure an endurance such as in the range of five years or more,additional corrosion protection is achieved by providing an additionallayer of fluoropolymer-treated carbon paper. The fluoropolymer preventsor inhibits the wetting of the carbon paper by the corrosiveelectrolyte, thus limiting the possibility of contact between themetallic part and the electrolyte.

The fluoropolymer-treated carbon paper is bonded to the metallic part(cooling plate 21 or current collector plate 28) which has beenprecoated with one of the above-mentioned metal coatings via the filmbonding process described above. An example of such process conditionsincludes hot pressing at 700 degrees F. for 45 minutes at a pressure ofabout 200 psi, followed by cooling to 400 degrees F. at the samepressure and further cooling to room temperature without pressure.

By utilizing this improved structure, the copper screen used in theprior art is not necessary and may be eliminated. The reason for this isthat silver and other noble metals do not normally form an electricallyinsulating film on their surface, as do copper or aluminum under manyenvironmental conditions. Since the function of the screen was in partat least, to break through the insulating film, the screen is no longerneeded. This in turn removes a target for corrosion and also results ina less vulnerable seal against the acid electrolyte. The excess resin isbeing compressed according to this invention directly into the pores ofthe treated carbon paper, thusfurther reducing its porosity and limitingthe possibility of contact between the electrolyte and the metallicpart.

Silver is advantageously employed as the metal coating due to itsrelatively low price. Samples prepared utilizing silver coating andfluoropolymer-treated paper showed the lowest corrosion current whencompared to a sample utilizing copper coating and fluoropolymer-treatedcarbon paper. Ruthenium is also preferred, in view of its relatively lowcost and the potential of superior corrosion protection observed in thetests.

A current collector 2 ft² was prepared according to this invention. Thecollector was provided with an electroplated metal coating of silverhaving a thickness of 0.25 mils. Fluoropolymer-treated carbon paper wasbonded to the collector using polyethersulfone as the resin, employinghot pressing. The same conditions and parameters for hot pressing asdiscussed above were used. The curent collector and interface wereincorporated into a fuel cell stack and tested for 1500 hours. Thecurrent collector was re-examined after the conclusion of this test. Nocorrosion was detected.

While carbon paper is a preferred conductive layer material, othermaterials which could be employed comprise wet proofed carbon paper,vitreous carbon, molded carbon plates, and corrosion resistant foilmaterial such as gold. While polyethersulfone is a preferred resinouslayer material, other resinous matrials which could be employed comprisepolyphenylsulfone, fluorinated polymers such as PTFE, fluorinatedethylene propylene,a nd perfluoralkoxy polymers and other corrosionresistant thermoplastic materials.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiments are therefore to be considered as inall respects illustrative and not restrictive, the scope of theinvention being indicated by the appended claims, and all changes whichcome within the meaning and range of equivalency being intended to beembraced therein.

We claim:
 1. A fuel cell stack comprising a plurality of stackedelements, including at least one corrosion resistant, electricallyconductive, fluid impervious interface arranged between two of saidelements which are not adjacent to one another, said interfacecomprising a non-cupreous metal coating formed on at least one of saidelements and a conductive layer bonded to at least said metal-coatedelement by a hot-pressed resin, said resin substantially filling anypores in said conductive layer.
 2. A fuel cell stack as in claim 1wherein one of said two elements comprises a gas distribution plate andwherein the other of said two elements comprises a metal-coated coolingplate.
 3. A fuel cell stack as in claim 1 wherein one of said twoelements comprise a gas distribution plate and wherein the other of saidtwo elements comprises a metal-coated current collecting plate.
 4. Afuel cell stack as in claim 1 wherein said conductive layer comprisescarbon paper and said resin comprises polyethersulfone.
 5. A fuel cellstack as in claim 4 wherein said conductive layer comprises carbon papertreated with a fluoropolymer.
 6. A fuel cell stack as in claim 1 whereinsaid metal coating is selected from the group consisting of silver,gold, ruthenium, ruthenium-nickel and palladium-nickel and combinationsthereof.
 7. A process for forming a corrosion resistant, electricallyconductive, fluid impervious interface between two adjacent elements ofa fuel cell cell comprising:(a) coating at least one of said elementswith a non-cupreous metal; (b) arranging between said elements aconductive layer and a resin layer between said conductive layer andsaid metal-coated element; and (c) hot pressing together said twoelements having said layers therebetween so that a bonded, bridgingelectrical contact is provided between said conductive layer and themetal-coated surface of at least one of said two elements and so thatsaid resin substantially fill the pores of said conductive layer.
 8. Aprocess as in claim 7 wherein said hot pressing is carried out at apressure of from about 100 to about 300 psi and at a temperature of fromabout 500 to about 750 degrees F.
 9. A process as in claim 8 whereinsaid conductive layer comprises carbon paper and said resin comprisespolyethersulfone.
 10. A process as in claim 7 wherein one of said twoelements comprises a gas distribution plate and wherein the other ofsaid two elements comprises a cooling plate.
 11. A process as in claim 7wherein one of said two elements comprises a gas distribution plate andwherein the other of said two elements comprises a current collectingplate.
 12. A process as in claim 7 wherein said metal is selected fromthe group consisting of silver, gold, ruthenium, ruthenium-nickel andpalladium-nickel and combinations thereof.